The broad objectives of this project are to study the participation of the contractile protein system in blood platelet function. The studies to be conducted will be concerned with mechanisms(s) by which actomyosin, under the effect of aggregating agents, activates and/or inhibits contraction, relaxation, secretion and aggregation. The interaction of each of these elements with contractile proteins will be studied in order to determine how they initiate, maintain and suppress each of the physiological functions of platelets that control normal hemostasis. Platelets in the presence of ADP undergo certain mechanochemical changes. One direct approach to elucidate some of the mechanisms involved is to determine ADP binding and the influence which it exerts on the dissociation and reassociation of the actomyosin in both the membrane and cytoplasm. c-AMP levels, suggested to play a key role in aggregation, may be linked to Ca 2 ion mobilization, therefore, studies will be carried out on the phosphorylation of a protein from sacroplasmic reticulum, and the involvement of c-AMP on the catalysis of this reaction and the influence on the sequestation of Ca2 ion by the microscome. The lower ATPase activity of platelet actomyosin, as compared to muscle, may be due to a missing light chain on the myosin molecule. Therefore, isolation, characterization and hybridization experiments are necessary to understand their biological functions. The ultimate aim is to investigate whether in certain deficiencies, such as Glanzmann thrombasthenia, an altered contractile mechanism is the cause of abnormal platelet function.